This invention relates to a circuit arrangement for the selection and the tuning of a radio-electric signal in a signal receiving set, comprising a controllable local oscillator whose frequency is controlled by a control loop which maintains it in a fixed relationship with a number N.
In a circuit arrangement of this type, suitable in particular for a television receiver, for selecting and tuning one of the various receivable television channels, the number N is obtained generally by the use of a memory of the read-only type (ROM) having a cell for each tunable television channel. In fact, the selection of each channel is carried out by means of one or more numbered push buttons on a control panel, whereby a particular number is caused to be sent to said read-only memory which, from the corresponding cell, generates another number which substantially determines said number N different for each channel. As can be seen from Table I at the end of this specification, in Europe there are today sixty selectable channels; to these channels have to be added five Italian channels. A further extension is foreseen for the future in view of the needs of cable television, and in fact additional channels, besides the sixty channels mentioned above, are already used in some countries (as for instance in Belgium). In the United States already today there are 82 channels. Therefore, it is necessary to provide memories with at least 80-90 cells of at least ten or even more bits each.
Such memories are necessarily very complex and expensive.